Latest Zello Update on iOS allows you to map Volume buttons as PTT!
You can see in the screen shot I was able to map the volume keys and it tested successful. I'm not sure if this is an oversight by Zello when they recently did a complete overhaul of Zello on the iOS platform. But for the time being, this feature exists again.
NOTE: The Camo skin has still not returned since the overhaul of the iOS Zello application, so if you are on an old version and really enjoy the camp skin, DO NOT update just yet. Only update if you really want to be able to map the PTT to the volume keys.
It's unclear right now if this feature is going to stick around or go away again. I'm afraid to ask Zello!
The great thing about this is you can be in the screen showing the users within a channel, or in the message history, and use the volume keys to key up. No longer do you have to be in the screen with the big mic circle to key up.
Also note: If you map both the volume keys as PTT as I do, you obviously won't be able to control the volume while in Zello. To control the volume, simply exit out of Zello, change the volume to the level you want, and then go back into Zello.